The present invention relates to a vehicle power steering system and more specifically to a hydraulic vehicle power steering system in which the resistance to actuation of a power steering control valve decreases with decreasing vehicle speed.
A know power steering control valve has a inner valve member which is coaxial with and rotatable relative to an outer valve member or sleeve. To effect actuation of a power steering motor to turn steerable vehicle wheels, the inner valve member is rotated relative to the outer valve member against the influence of a torsion bar. Since actuation of the control valve is resisted by only the torsion bar, the resistance to actuation of the valve does not vary as a function of vehicle speed. The construction of this known power steering control valve is described in U.S. Pat. No. 4,276,812 issued July 7, 1981 and entitled Power Steering Valve and Method of Making the Same.
It is known to increase the resistance felt by an operator of a vehicle to actuation of a power steering system as vehicle speed increases. One known way of accomplishing this is by having the supply of power steering fluid from a pump restricted as vehicle speed increases. However, in such a system, if a quick turn is attempted at high vehicle speeds, the restricted supply of fluid to the power steering motor may prevent the steering system from responding quickly to rapid rotation of the steering wheel.
An alternative means to increase the resistance felt by an operator of a vehicle to actuation of a power steering system is described in U.S. Pat. No. 4,373,598, issued Feb. 15, 1983 and entitled Auxiliary Steering for Motor Vehicles, which has the fluid pressure in a reaction chamber increase as vehicle speed increases. Increasing the fluid pressure in the reaction chamber increases the resistance to relative rotation between a pair of valve members. Power steering control valves in which the fluid pressure in a reaction chamber is increased to increase the resistance to relative movement between valve members are also disclosed in U.S. Pat. No. 4,034,825 issued July 12, 1977 and entitled Power Assisted Vehicle Steering and in U.S. Pat. No. 4,583,610 issued Apr. 22, 1986 and entitled Power Steering Apparatus.
These power steering systems rely on increasing fluid pressure in a reaction chamber to increase the resistance to relative movement between a pair of steering control valve members as vehicle speed increases. If there should be a loss of fluid pressure in the reaction chamber, the power steering control valve would be easily actuated. The resulting reduction in the force required to actuate the power steering control valve could result in excessive steering wheel rotation in the event of a loss of pressure in the reaction chamber at high vehicle speeds.